followship.rb

class Followship < ActiveRecord::Base
  attr_accessible :followed_id

  belongs_to :follower, :class_name => "User"
  belongs_to :followed, :class_name => "User"

  validates :follower_id, :presence => true
  validates :followed_id, :presence => true
end

user.rb

class User < ActiveRecord::Base

  has_many :followships, :foreign_key => "follower_id",
                         :dependent => :destroy
  has_many :following, :through => :followships,
                       :source => :followed

  def following?(followed)
    followships.find_by_followed_id(followed)
  end  

  def follow!(followed)
    unless following? followed
      followships.create!(:followed_id => followed.id)
    end
  end  

  def unfollow!(followed)
    followships.find_by_followed_id(followed).destroy
  end  
end